The present invention relates to a control system for a color copier which sets up an adequate color copying time for transfer papers of any particular size.
A typical color copier known in the art includes an optical system for scanning a document, a photoconductive drum, and a transfer drum located to face and in contact with the photoconductive drum. The photoconductive drum is sequentially exposed to a plurality of color components which are separated by the optical system and representative of a color document. Each of the resulting latent images is developed by a toner which is complementary in color to the latent image. The toner images are sequentially transferred to a transfer paper which is held on and rotated together with the drum. In this type of color copier, a prerequisite is that the optical system, photoconductive drum and transfer drum be driven in synchronism in order to eliminate the deviation of colors on the paper, which is detrimental to the quality of reproduction. To meet this requirement, there may be used a system in which a drive source of the photoconductive drum and that of the transfer drum are interconnected by gears and others which encounter a minimum of backlash, while the optical system is driven by a servo motor or the like which shows rapid response during speed control. However, a problem with such a system is that even when the size of papers is small a substantial period of time is necessary for a copy to be produced.
A control system capable of setting up an adequate copying time which matches itself to a transfer paper size is disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 60-218673. The system disclosed uses a scanning sensor responsive to a scan start position of the optical system, and a paper sensor disposed near the transfer drum to sense the trailing end of a transfer paper loaded on the drum. The times at which a transfer is started and ended are determined on the basis of the output signal of the scanning sensor and that of the paper sensor, respectively. During the interval between the times of the start and end of transfer determined, the rotation speed of the transfer drum is variably controlled to align the leading end of a transfer paper and that of each toner image representative of a particular color component.
Such a prior art system, however, cannot be accomplished without increasing the cost because the paper sensor responsive to the trailing end of a paper has to be associated with the transfer drum. Further, the accuracy of detection attainable with the paper sensor is limited and, therefore, the entire system lacks reliability.